Heat-resistant AL-CU-MG-AG alloy and process for producing a semifinished part or product composed of such an aluminum alloy

ABSTRACT

A heat-resistant Al—Cu—Mg—Ag alloy for producing semifinished parts or products, which is suitable for use at elevated temperatures and has good static and dynamic strength properties combined with an improved creep resistance and comprises: 0.3-0.7% by weight of silicon (Si), not more than 0.15% by weight of iron (Fe), 3.5-4.7% by weight of copper (Cu), 0.05-0.5% by weight of manganese (Mn), 0.3-0.9% by weight of magnesium (Mg), 0.02-0.15% by weight of titanium (Ti), 0.03-0.25% by weight of zirconium (Zr), 0.1-0.7% by weight of silver (Ag), 0.03-0.5% by weight of scandium (Sc), 0.03-0.2% by weight of vanadium (V), not more than 0.05% by weight of others, individually, not more than 0.15% by weight of others, total, balance aluminium, is described. A process for producing a semifinished part or product composed of the abovementioned aluminium alloy is also described.

The invention relates to a heat-resistant Al—Cu—Mg—Ag alloy for producing semifinished parts or products, suitable for use at rather high temperatures and with high static and dynamic strength properties combined with an improved creep resistance. The invention also relates to a process for producing a semifinished part or product composed of such an aluminum alloy.

An alloy of the above-cited type is known from EP 1 518 000 B1 from which semifinished parts are produced with high static and dynamic strength properties and a creep resistance that is improved in comparison to previously known, similar aluminum alloys. This alloy is registered with the Aluminum Association (AA) as alloy AA2016. This previously known alloy already approximately unites the strength properties necessary for semifinished parts and products that must resist high static and dynamic loads, which properties are known from the alloys AA2014, AA 2014A or AA2214 and have an improved creep resistance, that is: an improved resistance under the action of temperature. The alloy AA2016 therefore satisfies the claims put on semifinished parts and products produced from them that are exposed for a short time to elevated temperatures such as is the case, for example in the wheel halves of airplanes. These semifinished products are exposed to elevated temperatures only during braking after the airplane sets down on the landing strip.

The alloys AA2618 and AA2618A are considered to be especially creep-resistant. However, semifinished parts and products produced from these alloys have only relatively low static and dynamic strength values.

The alloys for producing semifinished parts with high static and dynamic strength properties in accordance with AA2014, AA2014A and AA2214 differ chemically from the alloys with long-time thermal stability according to AA2618 and AA2618A in particular in that the very strong aluminum alloys contain relatively high amounts of the elements silicon, copper and manganese and on the other hand relatively low amounts of the elements magnesium and iron whereas the previously described long-time thermally stable aluminum alloys have a reduced amount of silicon, copper and manganese in contrast to the above and on the other hand an elevated content of iron, nickel and magnesium. In addition, nickel is mixed into the long-time thermally stable alloys.

The alloy AA2016 differs from the previously described alloys in particular by an admixture of the element silver with amounts between 0.30 and 0.7 wt %. There are also differences in the remaining alloy elements in comparison to the composition of the previously cited, very strong aluminum alloy and relative to the previously cited aluminum alloys whose semifinished parts have a good creep resistance.

Even if the aluminum alloy AA2016 is already such a previously known one with which semifinished parts and products can be produced that satisfy high static and dynamic strength requirements and that in addition also resists elevated temperatures in short-time use, there has long been a desire to have available an aluminum alloy for producing semifinished parts and products that resist elevated temperatures not only in short-time use. Such requirements are placed on a plurality of products, for example, on the compressor wheels of a turbocharger in motor vehicle engine uses. These structural components must not only resist high static and dynamic loads but also the temperatures prevailing in such a use for the duration of the use. Similar requirements for long-time stability at rather high temperatures are also applicable to turbocharger compressors in large engines in ship construction.

Starting from this discussed prior art, the invention therefore has the task of suggesting an alloy from which a semifinished part or a product can be produced that satisfies the desired properties for static and dynamic strength as well as the long-time stability under influences of temperature.

This task is solved in accordance with the invention by a heat-resistant Al—Cu—Mg—Ag alloy for producing semifinished parts or products, suitable for being used at rather high temperatures, with high static and dynamic properties of strength in combination with an improved creep resistance, containing:

-   -   0.3 -0.7 wt % silicon (Si)     -   max. 0.15 wt % iron (Fe)     -   3.5-4.7 wt % copper (Cu)     -   0.05-0.5 wt % manganese (Mn)     -   0.3-0.9 wt % magnesium (Mg)     -   0.02-0.15 wt % titanium (Ti)     -   0.03-0.25 wt % zirconium (Zr)     -   0.1-0.7 wt % silver (Ag)     -   0.03-0.5 wt % scandium (Sc)     -   0.03-0.2 wt % vanadium (V)     -   max. 0.05 wt % others, individually     -   max. 0.15 wt % others, total     -   remainder aluminum.

This alloy has as particularity the alloy elements scandium and vanadium with the previously cited amounts. It is attributed to the interaction of these elements together with the elements titanium and zirconium on the one hand and to the silver contained in the alloy on the other hand that a semifinished part produced from this alloy and accordingly also the end product have sufficiently high static and dynamic properties of strength as well as an especially good creep resistance. The strength properties can be slightly reduced in comparison to those of semifinished parts from an aluminum alloy AA2016 but are clearly increased in comparison to such semifinished parts produced from the alloy AA2618. These special properties of a semifinished part produced from such an aluminum alloy were not to be expected. Therefore, this alloy is suitable for producing semifinished parts and products that not only have to satisfy high static and dynamic strengths but also must have a long-time stability under thermal influences, and therefore can have an excellent resistance to creep.

In an advantageous embodiment the alloy contains 0.08 to 0.2 wt % scandium and 0.10 to 0.2 wt % vanadium. In another specification of this alloy composition the aluminum alloy contains the electrodes titanium, zirconium, scandium and vanadium with the following amounts:

-   -   0.12 to 0.15 wt % titanium (Ti),     -   0.14 to 0.16 wt % zirconium (Zr),     -   0.13 to 0.17 wt % scandium (Sc) and     -   0.12 to 0.15 wt % vanadium (V).

Another improvement of the properties in question of a semifinished part or product produced from such an alloy can be achieved if care is taken that the sum of the elements zirconium, titanium, scandium and vanadium is less than or equal to 0.4 wt %, in particular less than or equal to 0.35 wt %.

The aluminum alloy preferably contains zirconium with amounts between 0.03 and 0.15 wt %. Titanium is preferably contained in the alloy with amounts between 0.03 and 0.09 wt %.

It is advantageous if the iron content of the alloy is limited to a max. of 0.09 wt %.

The special properties of the claimed Al—Cu—Mg—Ag alloy also appear if it has only a reduced amount of dispersoid producers. This is present, for example, if the claimed alloy comprises the following amounts of the elements titanium, zirconium, scandium and vanadium:

-   -   0.04 to 0.06 wt % titanium (Ti),     -   0.05 to 0.07 wt % zirconium (Zr),     -   0.08 to 0.10 wt % scandium (Sc) and     -   0.10 to 0.12 wt % vanadium (V).

The aluminum alloy preferably contains 0.3 to 0.6 wt % silver.

Silicon preferably participates in the buildup of the alloy properties between 0.3 and 0.6 wt %.

The manganese content of the aluminum alloy is preferably set at 0.1 to 0.3 wt %.

Another improvement of the special static and dynamic strength properties as well as of the creep resistance can be achieved if the content of the elements silicon, copper, manganese, magnesium and silver of the aluminum alloy is limited as follows:

-   -   0.45-0.55 wt % silicon (Si)     -   4.10-4.30 wt % copper (Cu)     -   0.15-0.25 wt % manganese (Mn)     -   0.5-0.7 wt % magnesium (Mg) and     -   0.40-0.55 wt % silver (Ag).

Investigations have shown that the alloy and the semifinished parts or products produced from it have an especially good creep resistance if the sum of the elements silver, zirconium, scandium and vanadium is at least 0.60 wt % and maximally 1.1 wt %.

It is advantageous if the elements silver and scandium are contained in the alloy in amounts such that the ratio of the silver amounts to the scandium amounts is between 5 and 23, preferably between 9 and 14.

The elements scandium and zirconium are advantageously contained in the alloy in a ratio between 1 and 17, preferably between 6 and 12.

As regards the elements silver and vanadium, a ratio of the silver amounts to the vanadium amounts between 0.5 and 14 is considered to be especially purposeful, in particular a ratio between 5 and 9.

Semifinished parts or products are typically produced from the previously cited heat-resistant aluminum alloy by the following steps:

(a) Casting of a bar from the alloy with sufficient dissolution of the electrodes zirconium, scandium and vanadium,

(b) Homogenization of the cast bar at a temperature that is as close as possible below the melting temperature of the alloy for a time that is sufficient for achieving the most uniform distribution possible of the alloy elements in the cast structure, preferably at 485 to 510° C. for a period of 10 to 25 h,

(c) Thermal deformation of the homogenized bar by extruding, forging (including reverse extrusion molding) and/or rolling in the temperature range of 280 to 470° C.,

(d) Solution annealing of the extruded, forged and/or rolled semifinished part at temperatures that are high enough to bring the alloy elements necessary for the hardening into solution distributed in the structure, preferably at 480 to 510° C. over a time of 30 min to 8 h,

(e) Quenching the solution-annealed semifinished part in water with a temperature between room temperature and 100° C. (boiling water) or in water-glycol mixtures with temperatures ≦50 ° C. and glycol contents of up to 60%,

(f) selective cold deformation of the quenched semifinished part by upsetting or stretching by an amount that results in a reduction of the intrinsic tensions produced during the quenching in cool quenching medium, preferably by 1-5%, and

(g) Thermal hardening of the semifinished part quenched in this manner and selectively cold-upset or stretched at temperatures adapted to the planned usage, preferably between 80 and 210° C. over a time of 5 to 35 h, preferably 10 to 25 h in a 1-, 2- or 3-stage process.

A sufficient dissolution of the electrodes zirconium, scandium and vanadium can therefore be achieved by moving the melt during the melting of the alloy before the casting step and during the casting of a bar. It is especially advantageous if the melt is moved by convection. Such a convection can be produced by external magnetic influences, for example, in an induction furnace. Therefore, the aluminum alloy is preferably melted in an induction furnace.

The invention is described in the following using exemplary embodiments, also in comparison to previously known aluminum alloys, with reference made to the attached figures. In the figures:

FIG. 1: shows a diagram with the chemical composition of the claimed alloy in comparison to the chemical compositions of previously known aluminum alloys,

FIG. 2: shows a comparison of the creep properties of the claimed alloy with a previously known alloy considered to be especially creep-resistant, and

FIG. 3: shows a Larsen-Miller diagram for representing the creep behavior of the claimed alloy in comparison to previously known ones.

FIG. 1 shows a comparison of the chemical composition of the claimed alloy with previously known aluminum alloys. On the one hand, those alloys are compared from which semifinished parts or products with high static and dynamic strength properties can be produced in a known manner. This concerns the alloys AA2014, AA2014A and AA2214. In addition, two previously known alloys are compared that are associated with an especially good long-time stability under thermal influences. This concerns the alloys AA2618 and AA2618A. The previously known alloy AA2016 is also given. The data given in the table for the amounts of the particular alloy elements are taken from the International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys, The Aluminum Association Inc., 1525 Wilson Boulevard, Arlington, April 2006.

The table of FIG. 1 indicates the alloy in accordance with the invention with “W”. The comparison of the alloy compositions clearly represents the differences of the claimed heat-resistant aluminum alloys by the addition of the elements vanadium and scandium and the special selection of the remaining alloy components including its particular amount. It is also clear from this comparison that the claimed alloy W cannot be derived as the sum or in some other manner from these previously known alloys.

Two typical alloy compositions of the claimed alloy were produced and investigated for the production of test pieces and for carrying out investigations of strength at room temperature and an elevated temperature. The two alloys W1 and W2 had the following chemical composition:

W1 W2 Element wt % wt % Si 0.51 0.50 Fe 0.092 0.084 Cu 4.06 4.22 Mn 0.186 0.207 Mg 0.591 0.586 Cr 0.009 0.013 Ni 0.002 0.009 Zn 0.009 0.007 Ti 0.128 0.059 Zr 0.146 0.059 V 0.131 0.115 Sc 0.137 0.089 Ag 0.46 0.49 Others individually 0.05 0.05 Others total 0.15 0.15 Al Remainder Remainder

Furthermore, test pieces of the comparison alloys AA2016 and AA2618 were produced and correspondingly investigated. Refer here regarding the theoretical composition of these alloys to the data in FIG. 1.

In order to determine the strength properties the alloys W1 and W2 were cast on an industrial scale to cast extrusion blocks with a diameter of 370 mm, whereby care was taken that the elements zirconium, scandium and vanadium were sufficiently dissolved during the casting of the bars. To this end the melt was put in motion by generating a convection in the melt. The cast extrusion blocks were homogenized in order to compensate the crystal segregations conditioned by the hardening. To this end of the blocks were homogenized and cooled off in two stages in a temperature range of 500° C. to 550° C. After the twisting off of the casting skin the homogenized blocks were preheated to approximately 400° C. and multiply deformed to freeform forged pieces with a thickness of 100 mm and a width of 250 mm. Subsequently, the freeform forged pieces from alloy W1 and W2 were solution-annealed at least 2 h at 500° C., quenched in water and subsequently hot-hardened between 165° C. and 200° C. Tensile tests were taken from the hot-hardened freeform forged pieces on which the strength properties were determined at room temperature in the longitudinal (L) test position. The results are listed in the table below:

R_(p0.2) R_(m) A₅ Alloy [MPa] [MPa] [%] 2016 446 490 11.1 2618 344 432 10.4 W1 399 449 8.1 W2 383 437 10.6

For purposes of a comparison the strength properties for freeform forged pieces of the alloys AA2016, followed by W1, W2 and AA2618 in the heat-hardened state are additionally indicated in the table.

The alloy AA2016 shows the greatest strength (stretch limit), followed by W1, W2 and AA2618. A sufficient ductility of >8% is achieved by all alloys. It should be especially emphasized at this point that the strength values of the comparison alloy AA2016 were not able to be reached with the test alloys W1, W2; however the test values achieved clearly exceed those of the other comparison alloy AA2618. For the cases of use in question the strength values that the test alloys W1, W2 have are sufficient. It is important that the test alloys W1, W2 have a significantly better creep resistance, as described in the following with reference made to FIG. 2, in comparison to the comparison alloy AA2618 considered to be creep-resistant.

The differences are especially noticeable in a comparison of the creep behavior of the alloy AA2618 known as creep-resistant with the alloy W2. This comparison is shown in FIG. 2. FIG. 2 shows in the diagram the creep properties of the particular alloy at 190° C. and a creep tension of 200 MPa. While the alloy AA2618, that is known as especially creep-resistant and has previously been used for such purposes, breaks already after about 320 hours in the prescribed test setup and experienced a plastic expansion of about 1% already at about 230 hours, the examined time of 500 h was not sufficient to make the test alloy W2 break. At the time of the break of the test piece of the alloy AA2618 a plastic deformation of only about 0.2% was able to be determined for the test alloy W2. The improved creep resistance of the claimed alloy in comparison to the alloy AA2618, that is considered to be especially creep-resistant, is surprising.

The test pieces of the other test alloy W1 have a creep resistance that corresponds to the one shown in FIG. 2 in the diagram using the test alloy W2.

The special properties of the claimed alloy are also evident by a comparison of this alloy and of the two test alloys W1, W2 with previously known alloys in a Larsen-Miller diagram. FIG. 3 shows such a diagram. In this representation the strength properties are shown linked with a temperature resistance. The alloy AA2618 previously known as especially creep resistant is distinguished by a relatively slight inclination of its break line. The alloy AA2014 on the other hand, that meets the high static and dynamic requirements, has a distinctly steeper angle of inclination of its break line. The curves of these two alloys intersect. That means that in the test structure documented in the diagram at first the alloy AA2214 resists higher tensions, namely, in the curve section located above the curve of the alloy AA2618, and decreases much more rapidly with increasing temperature and/or time as regards its breaking tension than the alloy AA2618. The alloy AA2016 is also entered in this diagram for comparison. Since this curve is located to the right of the curve of the alloy AA2014, it is clear that it is more long-time resistant in comparison to the alloy AA2014. It also becomes clear that the alloy AA2016 requires a higher tension up to a certain point in time in order to bring about a break.

These curves of previously known aluminum alloys are opposed by the area of the Larsen-Miller diagram in which the values of semifinished parts or products produced with the claimed alloy are located. The line of the test pieces of the test alloys W1 and W2 are concretely entered, whereby it is to be taken into consideration regarding this line representation that this line does not represent the break line but rather the state of the test samples after a test time of 500 hours. A break did not occur within this time (see also FIG. 2 in this regard by way of comparison). Therefore, the sketched-in lines are considered to be minimum lines as regards the test alloys W1, W2. The actual break lines of the test alloys W1, W2 are located much further to the right in the Larsen-Miller diagram. Even the inclination of these two curves should probably be significantly smaller than it is sketched in. For this reason the representation of a field was selected in order to be able to compare the improved properties of the claimed alloy with the properties of the previously known alloys discussed. The improved creep behavior of the claimed alloy can be clearly gathered from the Larsen-Miller diagram of FIG. 3. 

1. A heat-resistant Al-Cu-Mg-Ag alloy for producing semifinished parts or products, suitable for use at rather high temperatures and with high static and dynamic strength properties combined with an improved creep resistance, containing: 0.3-0.7 wt % silicon (Si) max. 0.15 wt % iron (Fe) 3.5-4.7 wt % copper (Cu) 0.05-0.5 wt % manganese (Mn) 0.3-0.9 wt % magnesium (Mg) 0.02-0.15 wt % titanium (Ti) 0.03-0.25 wt % zirconium (Zr) 0.1-0.7 wt % silver (Ag) 0.03 0.5 wt % scandium (Sc) 0.03-0.2 wt % vanadium (V) max. 0.05 wt % others, individually max. 0.15 wt % others, total remainder aluminum.
 2. The aluminum alloy according to claim 1, characterized in that it contains 0.12 to 0.15 wt % titanium (Ti), 0.14 to 0.16 wt % zirconium (Zr), 0.13 to 0.17 wt % scandium (Sc) and 0.12 to 0.15 wt % vanadium (V).
 3. The aluminum alloy according to claim 1 or 2, characterized in that the sum of the elements zirconium, titanium, scandium and vanadium is less than or equal to 0.4 wt %.
 4. The aluminum alloy according to claim 1, characterized in that it contains 0.04 to 0.06 wt % titanium (Ti), 0.05 to 0.07 wt % zirconium (Zr), 0.08 to 0.10 wt % scandium (Sc) and 0.10 to 0.12 wt % vanadium (V).
 5. The aluminum alloy according to one of claims 1 to 4, characterized in that it contains 0.45-0.55 wt % silicon (Si) 4.10-4.30 wt % copper (Cu) 0.15-0.25 wt % manganese (Mn) 0.5-0.7 wt % magnesium (Mg) and 0.40-0.55 wt % silver (Ag).
 6. The aluminum alloy according to one of the previous claims, characterized in that the sum of the elements silver, zirconium, scandium and vanadium is at least 0.60 wt % and maximally 1.1 wt %.
 7. The aluminum alloy according to one of the previous claims, characterized in that it contains the elements silver and scandium in a ratio of Ag: Sc=5−23.
 8. The aluminum alloy according to one of the previous claims, characterized in that it contains the elements scandium and zirconium in a ratio of Sc: Zr=1−17.
 9. The aluminum alloy according to one of the previous claims, characterized in that it contains the elements silver and vanadium with an Ag: V=0.5−14.
 10. The aluminum alloy according to one of the previous claims, characterized in that the aluminum alloy contains an iron content of max. 0.09 wt %.
 11. A process for producing a semi finished part or product from the aluminum alloy according to one of claims 1 to 10, characterized by the steps: (a) Casting of a bar from the alloy with sufficient dissolution of the electrodes zirconium, scandium and vanadium, (b) Homogenization of the cast bar at a temperature that is as close as possible below the melting temperature of the alloy for a time that is sufficient for achieving the most uniform distribution possible of the alloy elements in the cast structure, preferably at 485 to 510° C. for a period of 10 to 25 h, (c) Thermal deformation of the homogenized bar by extruding, forging (including reverse extrusion molding) and/or rolling in the temperature range of 280 to 470° C., (d) Solution annealing of the extruded, forged and/or rolled semifinished part at temperatures that are high enough to bring the alloy elements necessary for the hardening into solution distributed in the structure, preferably at 480 to 510° C. over a time of 30 min to 8 h, (e) Quenching the solution-annealed semifinished part in water with a temperature between room temperature and 100° C. (boiling water) or in water-glycol mixtures with temperatures ≦50 ° C. and glycol contents of up to 60%, (f) selective cold deformation of the quenched semifinished part by upsetting or stretching by an amount that results in a reduction of the intrinsic tensions produced during the quenching in cool quenching medium, preferably by 1-5%, and (g) Thermal hardening of the semifinished part quenched in this manner and selectively cold-upset or stretched at temperatures adapted to the planned usage, preferably between 80 and 210° C. over a time of 5 to 35 h, preferably 10 to 25 h in a 1-, 2- or 3-stage process.
 12. The process according to claim 11, characterized in that the melt is moved before the step of the casting of a bar and during the casting of the bar for the sufficient dissolution of the elements zirconium, scandium and vanadium.
 13. The process according to claim 12, characterized in that the melt is moved by convection.
 14. The process according to claim 13, characterized in that the melt is melted in an induction furnace. 